1. Field of the Invention
The present disclosure relates to a liquid crystal display, which prevents a blur on a display image by periodically varying a common voltage of the liquid crystal display panel, and a driving method thereof.
2. Discussion of the Related Art
An active matrix type liquid crystal display device displays moving images using thin film transistors (hereinafter, referred to as “TFTs”) as switching elements. In comparison with a cathode ray tube (CRT), the liquid crystal display device can have a smaller size. Thus, the liquid crystal display device is used as displays in portable information devices, office equipment, computers, televisions, etc., and hence is fast replacing the cathode ray tube.
In the liquid crystal display device, a blur may appear on a display image. To test display quality, as shown in FIG. 1, when blocks of black level data and black and white level data of a chessboard test pattern are displayed on a display image for a long time, a blur 20 appears as shown in FIG. 2. It is assumed that such a blur 20 is caused by the phenomenon that, as shown in FIG. 3, when an electric field is applied to a liquid crystal cell due to the dielectric anisotropy of liquid crystal, the ions in the liquid crystal cell are separated in the opposite direction to each other according to the polarity of the electric field. In the liquid crystal cell, ions of different polarities are accumulated in a pixel electrode and a common electrode. When a DC voltage is applied to a liquid crystal layer for a long time, the alignment film is deteriorated as the accumulated amount of the ions increases, and, as a result, the orientation characteristics of the liquid crystal is deteriorated. Due to this, when a DC voltage is applied to the liquid crystal display device for a long time, a blur is generated. To solve the problem of blurring, a method for developing a liquid crystal material having a low dielectric constant or a method for improving an orientation material or orientation method has been attempted. However, there has been another problem that it is not easy to develop materials, and if the dielectric constant of liquid crystal is low, the driving characteristics of the liquid crystal are deteriorated.
The polarity of a data voltage Vdata of liquid crystal cells is varied according to a relative potential difference with a common voltage Vcom as shown in FIG. 4. The common voltage Vcom is generated as a DC voltage. A blur 20 may occur due to electric charges 31 and 32 separated and accumulated according to the polarities of the data voltage Vdata and the common voltage Vcom. Such a blur 20 appears in an irregular pattern even between panels manufactured in the same manufacturing line, thus making it difficult to remove only by improvement of the process.